Thermal and electronic transport in La 0.7Sr 0.3−xAg xMnO 3 compounds between 50 and 450 K

Abstract

Temperature dependence of electrical and thermal conductivity of nanocrystalline La 0.7Sr 0.3− x Ag x MnO 3 ( x = 0.0 5 , 0.10, 0.20, 0.25) pellets prepared by pyrophoric method is reported between 20 and 300 K. Silver doping in LSMO influences strongly both the electrical and thermal conductivity. The metal–insulator transition temperature shows significant enhancement from 303 K ( x=0.05) to 363 K ( x=0.25). Electrical conduction in La 0.7Sr 0.3− x Ag x MnO 3 above T C is consistent with adiabatic small polaron hopping, while, in the ferromagnetic region ( T < T C ) the resistivity data shows a nearly perfect fit for all the samples to an expression containing, the residual resistivity, two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Heat conduction in this compound is dominated by phonons and electronic part is only ∼1% of the total λ. Lattice thermal conductivity ( λ ph) between 20 and 250 K, estimated using relaxation time approach gives an excellent agreement (within 2%) with the measured values, only when Umklapp scattering and scattering by two-level states are included along with phonon scattering by grain boundaries, sheet-like faults and point defects in the phonon relaxation time expression. Strengths of these phonon scattering rates are also estimated individually. Two-dimensional sheet-like faults appear to be the most important phonon scatterer in La 0.7Sr 0.3− x Ag x MnO 3 compounds.